The $150 Mauser can end up the $1500 Mauser if you want it to be. $150 is a good price to pay for a decent action, then you rebarrel it to your favorite caliber, re-stock it, throw in a new bolt handle, a new trigger group etc. etc.
My advice is to do as little as possible. Once you start modifying it, there's no end. Buy a Tupperware stock for it, and leave the heavy wood one home. See what it does with cheap standard production ammo (Rem Green Box for instance). That old surplus ammo may be your problem regarding accuracy. If you absolutely HAVE to re-blue it, try a bottle of Birchwood Casey cold blue and a q-tip. You might be pleasantly surprised. Be also very careful with surplus ammo; it may very well have corrosive primers.
I got into Mosin Nagants the same way a few years ago. The idea is the same. As long as you don't do much of anything to them, you have a nice heavy gun to take to the woods on a rainy day. Once you start turning the sow's ear into a silk purse, you'll quickly find the price tag rises well past the cost of a new production rifle. It used to be everyone wanted a custom rifle built on a Mauser action. That was because there were so few other ways of getting a good rifle. Nowadays, you can go to the store, buy a Savage 110 with and Accutrigger for relatively cheap and have a tack driver out of the box. No muss, no fuss.
Even the DIY guys are getting to wondering why they do it anymore. It used to be you'd get a milsurp lead sprayer for $5 bucks and take it into the shop, dump a couple of hundred into it and come out with a 4MOA rifle and you were the hit of the barber shop. Nowadays the WWII surplus has all been picked through and all the good ones have been taken. You've got a milsurp lead sprayers for $150 and up and $1000 later you're still stuck with a 4MOA rifle with $0 resale value and no end in sight trying to get it to where its driving tacks like you'd hoped.
Then again, having your picture taken with a buck and your $150 WWII lead sprayer is getting to be retro and fashionable. I have a picture out on the web of my son at 14 with an M1 Garand and a nice fat doe. It's ended up on countless websites and forums, including a French language D-Day site.